Electric solenoids have been used in automotive applications for some time, and a variety of control systems have been designed to control their operating currents. One reason for the variety of control systems is the variation of system source voltages and solenoid characteristics. Automotive applications typically involve an interface of a computer operating at 5 volts to a solenoid powered by a 12-volt or 24-volt power supply. The resistance of each solenoid must be relatively low to ensure that, even with the lowest reasonable system voltage applied, adequate current is developed to effectively attract an associated armature.
If a relatively high system voltage is applied to a solenoid having a low resistance, however, a higher than required current is developed; and the solenoid is forced to dissipate the excess energy as heat. As an example, if the voltage applied to a solenoid having a resistance of 3 ohms varies between 7 and 30 volts, the power varies between 16 and 300 watts. Producing a solenoid that is simultaneously capable of being actuated by a low system voltage and capable of dissipating the excess energy generated by a high system voltage is difficult and costly, and the resulting solenoid is likely to be impractically large for many planned applications.
A common method of controlling current applied to a solenoid is that of using a linear control system. With a linear control system, however, a solenoid driver in the control system absorbs excess energy not required to drive the solenoid. A disadvantage of such a system is that it supplies more energy to the solenoid driver and to the solenoid than is needed, for example, with a switch-mode control system.
High-side solenoid drivers, that is, solenoid drivers disposed on the power supply side of a solenoid, are sometimes used in solenoid control systems. They require the attendant use of level shifting circuitry, however, which adds cost and complexity. As the circuitry becomes more sophisticated, the cost and complexity of associated interface circuitry also increases. Another disadvantage is the fact that semiconductors (for example, NPN bipolar transistors and N-channel field-effect transistors) used in control systems using low-side solenoid drivers are available in a wider selection with better characteristics at lower cost than are those used in high-side solenoid drivers.
While the foregoing function with a certain degree of efficiency when used in connection with the control of electric solenoids, they do not provide the advantages of the improved method and apparatus of the present invention as is hereinafter more fully described.